Re: [PATCH GCC]Improve bound information in loop niter analysis

[Jeff Law <law@redhat.com>]

On 07/28/2015 03:36 AM, Bin Cheng wrote:
> Hi,
> Loop niter computes inaccurate bound information for different loops.  This
> patch is to improve it by using loop initial condition in
> determine_value_range.  Generally, loop niter is computed by subtracting
> start var from end var in loop exit condition.  Moreover, loop bound is
> computed using value range information of both start and end variables.
> Basic idea of this patch is to check if loop initial condition implies more
> range information for both start/end variables.  If yes, we refine range
> information and use that to compute loop bound.
> With this improvement, more accurate loop bound information is computed for
> test cases added by this patch.
>
> Is it OK?
>
> Thanks,
> bin
>
> 2015-07-28  Bin Cheng<bin.cheng@arm.com>
>
> 	* tree-ssa-loop-niter.c (refine_value_range_using_guard): New.
> 	(determine_value_range): Call refine_value_range_using_guard for
> 	each loop initial condition to improve value range.
>
> gcc/testsuite/ChangeLog
> 2015-07-28  Bin Cheng<bin.cheng@arm.com>
>
> 	* gcc.dg/tree-ssa/loop-bound-1.c: New test.
> 	* gcc.dg/tree-ssa/loop-bound-3.c: New test.
> 	* gcc.dg/tree-ssa/loop-bound-5.c: New test.
>
>
> improve-loop-bound-analysis-20150728.txt
>
>
> Index: gcc/testsuite/gcc.dg/tree-ssa/loop-bound-3.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/tree-ssa/loop-bound-3.c	(revision 0)
> +++ gcc/testsuite/gcc.dg/tree-ssa/loop-bound-3.c	(revision 0)
> @@ -0,0 +1,22 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-ivopts-details" } */
> +
> +int *a;
> +
> +int
> +foo (unsigned char s, unsigned char l)
> +{
> +  unsigned char i;
> +  int sum = 0;
> +
> +  for (i = s; i > l; i -= 1)
So is this really bounded by 254 iterations?  ISTM it's bounded by 255 
iterations when called with s = 255, l = 0.   What am I missing here? 
Am I mis-interpreting the dump output in some way?

Similarly for the other tests.

Jeff